1 Field of the Invention
The present invention relates to a method for diffusing metal particles in a composite layer comprising a dielectric matrix and said metal particles.
The fields of application of the present invention especially comprise microelectronics, photovoltaics, optics, catalysis, sensors.
2 Description of Related Art
Thin layers, or matrixes, comprising metal particles and especially metal nanoparticles, have characteristics specific to their structure and composition. According to cases, the composite layers may thus have electric, catalytic, optical, or magnetic properties, for example.
In order to envisage various applications, especially in the above-mentioned fields, a control of the structure of the thin layers comprising metal particles is important to anticipate the final properties. For example, composite layers may have a surface plasmon resonance effect reinforcing the absorption of light and thus influencing the efficiency of a photovoltaic cell. The characteristics of the composite layer thus obviously need to be controlled.
Such composite layers may be obtained by a method of composite deposition on a substrate. The deposition is generally performed according to techniques known by those skilled in the art, that is, wet (impregnation, sol-gel) or dry (PVD, CVD) processes.
PVD is a physical vapor deposition by evaporation in vacuum while CVD is a chemical vapor deposition.
The composite layers thus obtained may have a homogeneous or multilayered structure, according to the preparation mode used.
When the matrix and the metal particles are deposited simultaneously, the particles arc embedded in the matrix, a small ratio of said particles appearing at the matrix surface. Such a deposition is homogeneous, especially across the thickness of the thin film or layer.
However, when the deposition is performed in several distinct successive steps, a multilayered architecture is obtained. In such an architecture, also called “sandwich” structure, the matrix deposition and the metal particle deposition are performed sequentially. Thus, the metal particles may be isolated within the matrix if the final layer is a matrix deposition.
Although these two methods remain quite distinct from each other and the depositions thus obtained each have their own characteristics, the metal particles may evolve in both cases. They may change size and shape and diffuse during the forming process. The control of the structure of the composite layer and thus of its properties thus becomes very random, especially in terms of repeatability.
It is established that an after-treatment of composite layers enables to modify or to control the particle shape, size, and distribution. It thus becomes possible to control the final characteristics and properties of composite layers.
Prior art comprises several solutions implementing an after-treatment. Pivin et al. have shown that the shape and the size of metal nanoparticles can be modified by ion irradiation (“Competing processes of clustering and mixing of noble metal film embedded in silica under ion irradiation”, J. C. Pivin, G. Rizza, Thin Solid Films, Vol. 366, pp. 284-293, 2000).
Teeng et al. have described the preparation of a nanocomposite layer TaN—Ag by co-sputtering. The growth of silver nanoparticles is controlled by an annealing (“Rapid Thermal Annealing”) (“Microstructural analysis and mechanical properties of TaN—Ag nanocomposite”, C. C. Tseng, J. H. Hsieh, S. C. Jang, Y. Y. Chang, W. Wu, Thin Solid Films, Vol. 517, pp. 4970-4974, 2009).
The optical absorption of surface plasmons after annealing between 400 and 800° C. of composite SiO2—Ag—Au films deposited by co-sputtering has been studied by Sangpour et al. (“The effect of Au/Ag ratios on surface composition and optical properties of co-sputtered alloy nanoparticles in Au—Ag—SiO2 thin film”, P. Sangpour, O. Akhavan, A, Z. Moshfegh, Journal of Alloys and Compounds, Vol. 486, pp. 22-28, 2009). It has been observed that the annealing causes the diffusion of metal elements, the forming of metal nanoparticles being detected by optical absorption.
Besides, Wang et al. Have described the photodiffusion of silver particles deposited on GeS under a Xe lamp (“Photodiffusion of silver in germanium-sulfur compounds studied by AFM, nanoindentation and RBS methods”, R. Wang, J. H. Horton, Physical Chemistry Chemical Physics, Vol. 5, pp.4335-4342, 2003). However, this document does not mention the presence of nanoparticles.
The present invention especially relates to a method of diffusion of metal particles in a composite layer, at ambient temperature and at decreased pressure, enabling to control the structure of the composite layer formed by the metal particles and a dielectric matrix.